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United States Patent |
6,063,147
|
Winter
,   et al.
|
May 16, 2000
|
Gasification of biosludge
Abstract
The high intracellular water content contained within the cell walls of the
bacterial cells of a dewatered biosludge is reduced by removing the
intracellular water in a denaturing operation. This operation comprises
heating the biosludge at a temperature sufficient to weaken the bacterial
cell walls. The weakened cell walls are then exposed to a reduced pressure
sufficient to form vapor within the cell and to thus rupture the weakened
cell walls and thereby release the intracellular water in the form of a
hot aqueous vapor and/or released intracellular water. The water-reduced
concentrated biosludge can then serve as a fuel source in a partial
oxidation reaction for the production of synthesis gas.
Inventors:
|
Winter; John D. (Independence, KS);
Richter; George N. (San Marino, CA)
|
Assignee:
|
Texaco Inc. (White Plains, NY)
|
Appl. No.:
|
213683 |
Filed:
|
December 17, 1998 |
Current U.S. Class: |
44/597; 44/593; 44/598 |
Intern'l Class: |
C10L 005/46; C10L 005/48 |
Field of Search: |
44/597,598
|
References Cited
U.S. Patent Documents
4396513 | Aug., 1983 | Haldeman | 210/734.
|
4668388 | May., 1987 | Dibble et al. | 210/150.
|
4702745 | Oct., 1987 | Kamei et al. | 44/10.
|
5250080 | Oct., 1993 | Michelena et al. | 44/575.
|
5431702 | Jul., 1995 | Schulz | 44/552.
|
5858222 | Jan., 1999 | Shibata et al. | 210/177.
|
Primary Examiner: McAvoy; Ellen M.
Attorney, Agent or Firm: Delhommer; Harold J.
Rodman & Rodman
Claims
What is claimed is:
1. A method for concentrating a biosludge for use as a fuel source for a
partial oxidation gasification reaction, wherein said biosludge comprises
a plurality of bacterial cells with cell walls containing intracellular
water, comprising:
(a) dewatering the biosludge to a dry solids content of at least about 3
weight %;
(b) heating the dewatered biosludge in a biosludge evaporator at a
temperature sufficient to weaken the cell walls containing the
intracellular water and at a rapidly reduced pressure sufficient to
rupture the weakened cell walls and release the intracellular water; and
(c) evaporating the released intracellular water as a hot aqueous vapor
from the ruptured cells of the biosludge, to produce the concentrated
biosludge fuel source for the partial oxidation gasification reaction.
2. The method of claim 1, wherein the dewatered biosludge is heated by
being directly contacted with a hot oil to form a hot oil/biosludge
mixture.
3. The method of claim 1, wherein the heating temperature used to weaken
the cell walls of the dewatered biosludge varies from about 80.degree. C.
to about 350.degree. C.
4. . The method of claim 1, wherein the rupture pressure varies from about
0 psia to about 60 psia.
5. The method of claim 1 wherein the temperature for evaporating the
released intracellular water varies from about 105.degree. C. to about
250.degree. C.
6. The method of claim 1, wherein the hot aqueous vapor is condensed and
used as a moderator in a partial oxidation gasification reaction.
7. The method of claim 2, wherein the hot oil/biosludge mixture is
subjected to sufficient temperature and pressure to weaken, rupture, and
release the intracellular water from the biosludge, thereby producing a
hot concentrated biosludge/oil mixture.
8. The method of claim 2, wherein the dewatered biosludge is preheated by
indirect contact with the hot aqueous vapor released from the ruptured
cells of the biosludge before being contacted with the hot oil.
9. The method of claim 7, wherein the ratio of hot oil to biosludge varies
from about 1:1 to about 15:1 respectively.
10. The method of claim 7, wherein the concentrated biosludge/oil is
combined with a sufficient amount of a hydrocarbonaceous material to form
a fuel for a partial oxidation gasification reaction.
11. The method of claim 10, wherein the ratio of concentrated oil/biosludge
to the hydrocarbonaceous fuel varies from about 1.50 to about 1:1.
12. The method of claim 10, wherein the hydrocarbonaceous material is an
oil having a heating value of at least about 8,000 BTU/pound.
Description
BACKGROUND OF THE INVENTION
Wastewater treatment facilities separate the wastewater components into
coarse solids, scum grit and sludge. Sewage sludge is a mixture of
suspended, colloidal and dissolved organic and inorganic matter which is
separated from wastewater during treatment.
Wastewater is generally subjected to a primary treatment wherein the
suspended solid content is removed by physical means such as screening and
gravity sedimentation. Chemical precipitation is useful in removing
lightweight suspended and colloidal solids.
The remaining liquid sewage is then subjected to a secondary treatment
wherein microorganisms, primarily bacteria, are used to stabilize and
denature waste components by degrading complex organics and/or killing
pathogens. The mixture of microorganisms is usually referred to as
"biomass." During the biological treatment of wastewater or sewage, the
waste components function as nutrients for the microorganisms, enabling
them to reproduce and multiply as they stabilize and denature the waste
components.
Thus, the quantity of biomass in a waste treatment system increases during
the stabilization and denaturing treatment. In order to avoid the buildup
of an excessive amount of microorganisms which can "choke" the process, a
portion of the microorganisms must be removed or "wasted" from the
treatment system. The wasted microorganisms are referred to as
"biosludge." A major cost component of all biologically based processes is
the need to dispose of this biosludge in an environmentally acceptable
fashion.
The general treatment or management of sludge involves stabilization of
biodegradable organics, concentration and dewatering, and ultimate
disposal of the stabilized, dewatered residue.
Generated sludges are often dilute, on the order of about. 1-2 percent
solids by weight. In order to reduce the volumetric loading on other
processes, the first step in sludge processing is often concentration, by
such means as gravity thickening and flotation.
Organic sludges from primary treatment can usually be concentrated to about
5-8 weight percent solids. Sludges from secondary treatment can usually be
gravity thickened to about 2 to 4 weight percent solids.
Dewatering is different from concentration in that concentration still
leaves the sludge with the properties of a liquid. Dewatering uses
mechanical operations such as centrifugation, vacuum and/or pressure
filtration and sand beds to produce a product which is essentially a
friable solid. When the water content of sludge is reduced by dewatering
to about 65-80 percent, it forms a porous solid called sludge cake. There
is no free water in the cake as the water is chemically combined with the
solids or tightly adsorbed on the internal pores or held within the cells
of microorganisms.
Biosludge is the organic biomass remaining after biological waste water
treatment. Biosludge usually has to be mechanically dewatered to reduce
water content below approximately 96 weight %. A significant source of
water in the biosludge is contained within the cells of biota present in
the sludge, and is referred to as "intracellular water." by dewatered
biosludge filter cake with no free liquid can still have a water content
in excess of 80% by weight due principally to the amount of intracellular
water contained in the cells of the dewatered biosludge.
"Dry solids" is the water-free residue left after a dewatered sample is
dried in nitrogen at 105.degree. C. until no further weight loss is
observed. The term "free liquid" is liquid that is not physically adsorbed
or encumbered or chemically combined, and can be released through
conventional filtration processes.
Currently available technologies for reducing the water content of the
biosludge and utilizing the biosludge are costly and require excessive
amounts of energy. In particular, they have all been round to be
uneconomical and impractical means for converting biosludge and sewage
sludge into a viable feed for a partial oxidation gasification reaction.
SUMMARY OF THE INVENTION
The high intracellular water content contained within the cell walls of the
bacterial cells of a dewatered biosludge is reduced by removing the
intracellular water in a denaturing operation. This operation comprises
heating the biosludge at a temperature sufficient to weaken the bacterial
cell walls. The weakened cell walls are then exposed to a reduced pressure
sufficient to form vapor within the cell and to thus rupture the weakened
cell walls and thereby release the intracellular water as free water or in
the form of a hot aqueous vapor. The water-reduced concentrated biosludge
can then serve as a fuel source in a partial oxidation reaction for the
production of synthesis gas.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a simplified schematic representation of the biosludge
concentration treatment.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In accordance with the present invention the heat values contained in
biosludge can be used as a simple and effective fuel source in a partial
oxidation process for the production of synthesis gas. The biosludge is
usually combined with a supplemental hydrocarbonaceous fuel such as coal
or oil, and undergoes cogasification in a partial oxidation reaction to
generate synthesis gas, also referred to as "syngas".
The partial oxidation gasification reaction is conducted under reaction
conditions that are sufficient to convert a desired amount of fuel or
feedstock to synthesis gas. Reaction temperatures typically range from
about 900.degree. C. to about 2,000.degree. C., preferably from about
1,200.degree. C. to about 1,500.degree. C. Pressures typically range from
about 1 to about 250 atmospheres, preferably from about 10 to about 200
atmospheres and most preferably about 20 to 80 atmospheres. The average
residence time in the reaction zone generally ranges from about 0.5 to
about 20, and preferably from about 1 to about 10 seconds.
The syngas reaction product leaving the partial oxidation reactor generally
includes CO, H.sub.2, steam, CO.sub.2, H.sub.2 S, COS, CH.sub.4, NH.sub.3,
N.sub.2, volatile metals and inert gases such as argon. The specific
product composition will vary depending upon the composition of the
feedstock and the reaction conditions. Non-gaseous byproducts include
particulate materials, generally carbon and inorganic ash.
The efficiency and economic viability of the biosludge cogasification
process requires the solids content of the biosludge be increased to as
high a level as possible before being combined with the supplemental
hydrocarbonaceous fuel and introduced into the partial oxidation reactor
for the production of syngas. It is important to maximize the ratio of the
biosludge to the supplemental fuel. Otherwise, the fuel loading can be
hindered by the high water content and/or poor slurrying properties of the
biosludge. Thus, the efficiency of the gasification of biosludge increases
as the water content of the biosludge decreases and the solids content
increases. A suitable ratio of biosludge to hydrocarbonaceous fuel on a
water-free weight basis is about 1:50 to about 1:1, and preferably about
1:25 to about 1:2, respectively.
The biosludge or biosolids fed to the dewatering process can be at any
water content. Thus, the biosludge can be sourced from the material
produced by simple settling in a primary treatment facility, or from
mechanically dewatered feed, and the like. The initial water content will
influence the amount of water removed in relation to the amount of "dry"
solids produced and the size of the heat exchangers. It is preferred to
use a biosludge feed to the dewatering process containing greater than 3%
by weight dry solids.
In one embodiment of the invention, the biosludge feed is preheated by heat
exchange with a heat source prior to the evaporation step. Generally there
will be large amounts of low grade excess heat available for heat exchange
from the syngas cooling train comprising the syngas trim cooler, syngas
cooler, flashgas condenser, or quench water coolers. A, particularly
desirable source of heat for the preheating step is the water vapor
exiting the biosludge evaporator.
The biosludge feed material, with or without preheating by heat exchange
with the water vapor exiting the biosludge evaporator, is combined with a
hot oil stream exiting the reboiler of a biosludge evaporator. The
biosludge evaporator is typically a flash evaporator. The relative amount
of hot oil to biosludge on an as-fed basis in the combined stream to the
biosludge evaporator varies from about 1:1 to about 20:1, and preferably
about 5:1 to about 15:1, respectively, by weight.
The temperature and pressure of the contacting step is regulated so that no
boiling of water occurs in the combined biosludge/oil stream prior to
entry into the biosludge evaporator. It is important that no boiling occur
at this point because once boiling commences, the volume expansion is
rapid and of great magnitude. For example, one pound of steam has
approximately 1,000 times the volume of one pound of water. Thus velocity
and pressure drop increase radically unless ample space is provided in the
evaporator to separate the vapor from the rest of the biosludge, oil
mixture. Cell rupture is enhanced by rapidity of the pressure drop.
The hot oil and biosludge enter the evaporator through a pressure reducing
device. In the evaporator, water vapor is released from the intracellular
water content of the biosludge. The pressure reducing device is typically
a valve located on the exterior inlet to the evaporator. As a result of
the contacting of the hot oil with the biosludge and the rapid pressure
reduction, the cell structure housing the intracellular water is weakened.
The weakened cell walls housing the intracellular water are then ruptured
in the evaporator, releasing the intracellular water. Some or all of the
intracellular water then vaporizes since the temperature of the
biosludge/oil mixture entering the evaporator is above the water
saturation temperature at the flash evaporator pressure.
Suitable pressures in the evaporator vary from about 0 about 60 psia, and
preferably about 0.5 to about 20 psia. Suitable temperatures of the
biosludge/oil mixture prior to the pressure reduction step are about
80.degree. C. to about 350.degree. C., and preferably about 90.degree. C.
to about 250.degree. C. As the water evaporates, the overall temperature
will drop. This is referred to by those skilled in the art as an
"adiabatic flash evaporation." The temperature in the evaporator and
fraction of water evaporated will be such that enthalpy of the entering
stream will equal the enthalpy of the streams leaving the evaporator
except for minor enthalpy or heat losses from walls of the vessel.
Water vapor exits the top of evaporator. The hot oil and concentrated
biosludge mixture exit the bottom of the evaporator. A portion of the
concentrated biosludge/oil mixture is passed to the gasifier. The
remainder is heated in a reboiler to provide the energy to convert the
intracellular water released in the evaporation step to vapor. A separate
stream of hot oil can be used to heat the concentrated biosludge mixture
which passes through the reboiler. Additional oil can be added to the
concentrated biosludge mixture when needed, preferably at the heating oil
inlet to the reboiler.
The hot oil and concentrated biosludge mixture that is heated in the
reboiler is then directly contacted with the biosludge feed material to
form a combined stream. The hot oil and concentrated biosludge mixture
provide sufficient heat to the combined stream to evaporate intracellular
water from the biosludge feed material in the evaporator.
The overhead vapor from the evaporator can be cooled by heat exchange with
the incoming biosludge or oil. The overhead vapor can then be further
cooled in a separate condenser or in a flash gas condenser. The water
condensed from the vapor can be used as a moderator in the partial
oxidation gasification reaction. Excess water can be routed to a
wastewater treatment plant to remove suspended and dissolved organic
substances prior to the discharge of the water in accordance with
environmental regulations.
The reboiler can be heated with external steam or fuel or can use some or
all of the heat sources available from the partial oxidation gasification
system depending on the energy available. The partial oxidation
gasification system includes syngas trim coolers, syngas coolers, flashgas
condensers, or quench water coolers. The amount and sources of heat used
will depend on the desired ratio of biosludge to oil and the water content
of the biosludge or sewage used.
Referring now to FIG. 1, biosludge stream 2 enters the heal exchanger 4
where it comes into indirect contact with hot water vapor stream 6 exiting
the top of the biosludge evaporator 8. The hot water vapor stream 6
indirectly preheats the biosludge 2 to a temperature of about 35.degree.C.
to about 250.degree. C. Cooled water vapor stream 10 which can also be a
mixture of liquid and vapor, exits heat exchanger 4 at a temperature of
about 25.degree.C. to about 120.degree. C., and enters the condenser 12
where it is divided into water stream 13, which can serve as a moderator
for the partial oxidation gasification reaction (not shown), and water
stream 14 which can be recycled to a waste water treatment plant (not
shown).
Heated biosludge stream 16 exits the heat exchanger 4 at a temperature of
about 30.degree. C. to about 240.degree. C., and is contacted directly
with the hot oil stream 18 exiting the reboiler 20 at a temperature of
about 80.degree. C. to about 350.degree. C. to form combined oil/sludge
stream 22 at a temperature of about 105.degree. C. to 250.degree. C. The
hot oil stream 18 also contains a portion 30 of concentrated biosludge
from line 24 exiting from evaporator 8. The combined oil/sludge stream 22
passes through the pressure reducing device 23 which is typically a valve
that causes the pressure of stream 22 to drop as it enters the biosludge
evaporator 8. The pressure is typically below the saturation pressure of
water at the temperature of stream 22, for example, about 0.01 atmospheres
(0.147 psia) to about 2 atmospheres (29.4 psia). The biosludge stream 16
contacts the hot oil stream 18 at a pressure which exceeds the water
saturation pressure at the temperature of stream 22. This prevents the
evaporation of water until stream 22 passes through the pressure reducing
device 23. The pressure can vary from about 0.1 atmospheres (1.47 psia) to
about 40 atmospheres (587.8 psia).
The hot oil stream 18 can be any oil with a heating value above about 8,000
BTU/pound. Typical examples of suitable oils include heavy crude oil, fuel
oil, atmospheric resid, vacuum resid, visbreaker tar, solvent deasphalting
residuum, or a combination of these oils.
The biosludge evaporator 8 operates at conditions which are designed to
rupture the bacterial cells containing the intracellular water of the
biosludge in stream 22. The chemical nature and temperature of the hot
oil, on the order of about 80.degree. C. to about 350.degree. C., acts to
weaken or disrupt the cell walls of the biosludge housing the
intracellular water.
The reduction of pressure from above the water saturation pressure at the
point of mixing of streams 18 and 16 to below the water saturation
pressure in the evaporator 8 causes a portion of the intracellular water
to vaporize, resulting in the rupturing of most of the bacterial cell
walls of the biosludge previously weakened due to the effect of the hot
oil on the cell membranes. The intracellular water is released as vapor in
stream 6 which exits from the top of evaporator 8.
The hot oil and concentrated biosludge stream 24 which remains after the
release of the intracellular water exits the evaporator 8 at a temperature
of about 105.degree. C. to about 250.degree. C.
The concentrated biosludge/oil stream 24 exiting the bottom of the
biosludge evaporator 8 is divided into oil/sludge streams 25 and 30.
Supplemental oil feed 26 from reservoir 40 can be combined as needed with
the oil/sludge stream 25 to form cogasification stream 28 which is
introduced into a partial oxidation gasification system (not shown) and
used as fuel for a partial oxidation reaction for the production of
synthesis gas.
Ultimately, oil has to be added to the process to make up for losses of oil
in stream 25. This oil can be added in stream 42 upstream of the pump 32
or in stream 27 downstream of the pump 32, with the choice being dictated
by design preferences and the temperature and the pressure of available
oil.
The additional oil feed stream 42 from oil feed reservoir 40 can be
combined with oil/sludge stream 30 to form combined stream 44 which is
passed through the pump 32 and enters the reboiler 20. If the oil in
reservoir 40 is available at sufficiently high pressure it can be added
downstream of pump 32 in stream 27 instead of upstream in stream 42.
The ratio of the biosludge dry solids contained in stream 24 to the oil in
streams 27 and 42 varies from about 0.01:1 to about 1:1, and preferably
from about 0.1:1 to about 0.99:1.
The reboiler 20 can be heated with steam, flashgas, or hot water, or hot
syngas introduced as stream 34 to provide the heat source for the reboiler
20, which exits as condensate, water, or cooled syngas stream 36.
The sources of heat used will depend on the ratio of biosludge to oil
desired and the water content of the biosludge used.
The ratio of oil/sludge stream 25 to concentrated oil/sludge feed 30 can
range from about 1:1 to about 1:100, and preferably about 1:2 to about
1:50 respectively.
All parts and percentages indicated throughout the application are by
weight, unless otherwise indicated.
Example
300 lbs of biosludge containing 4 weight % dry solids is treated in the
operating system of FIG. 1. After being preheated, the biosludge is
combined with 3100 lbs of a mixture comprising 2743.4 lbs oil, 329.2 lbs
dry solids and 27.4 lbs water. The heated combined biosludge/oil mixture
then undergoes pressure reduction and evaporation at 1.4 atmospheres
(20.58 psia) which results in the rupture of the bacterial cell walls of
the biosludge thereby releasing 287 lbs of intracellular water in the form
of a not aqueous vapor and producing 3113 lbs of a concentrated
biosludge/oil mixture which exits the evaporator. The concentrated
biosludge/oil mixture is divided into a first stream of 113 lbs of
biosludge/oil to provide a cogasification fuel stream for a partial
oxidation reaction.
The remaining 3000 lbs of the concentrated biosludge/oil mixture are
combined with 100 lbs of oil and pumped to the reboiler where the
combination is heated to 350.degree. C. and combined with incoming
biosludge.
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